Re: [boost] [integer] Promotion of endian library code from vault

From: Beman Dawes (bdawes_at_[hidden])
Date: 2008-05-29 21:51:07

Neil Mayhew wrote:
> On 28/05/08 04:25 PM Beman Dawes wrote:
>> Neil Mayhew wrote:
>>> ... have a base class that contains everything except the 
>>> constructors, and a derived class that adds the constructors
>> There are some issues with the base class approach. For it to be a 
>> POD, it can't have constructors, base classes, or private members.
> 
> Strictly speaking, yes, although we are only concerned about memory 
> layout here, so the class doesn't have to be a true POD. The other 
> concern is for the class to be able to live inside a union, and I think 
> the constructors are the the only thing stopping that. (I #if'd the 
> constructors and I was then able to use endian in a union.)

I was wrong above - you are correct that just the constructs have to be 
removed. Private members and base classes are OK.

>> It might be better to just have two separate types, endian and 
>> old_endian. (old_endian isn't very clear as a name, but calling it 
>> pod_endian becomes invalid the moment C++0x ships.)
>>
>>> The typedefs to big32_t etc. would then be #if'd to correspond to the 
>>> POD or non-POD classes as desired. I think this is better than 
>>> putting the #if's around the constructors themselves.
>> Hummm... Seems too obscure. Better to have two sets of typedefs, with 
>> the C++03 set prefixed by old_ (or whatever better name anyone can 
>> come up with.)
> 
> I don't like "old_", at least not if it has to appear in my code! :-)
> 
> I would like my client code to remain essentially the same when I 
> upgrade to a C++0x compiler. This means having the same type names, 
> which is why I suggested conditional typedefs. I thought this was 
> cleaner than putting the conditionals inside the endian class itself. 
> However, it doesn't make a lot of difference.
> 
> Perhaps the simplest and best solution is therefore:
> 
>     class endian< ... >
>       : cover_operators< ... >
>     {
>       public:
> #if defined(CXX_0X) || !defined(ENDIANS_IN_UNIONS)
>         endian() {}
>         endian(T val) { ... }
> #endif

That would work. The macro names need tweaks. Maybe

!defined(BOOST_NO_RELAXED_PODS) || !defined(BOOST_ENDIANS_IN_UNIONS)
or
!(defined(BOOST_NO_RELAXED_PODS) && defined(BOOST_ENDIANS_IN_UNIONS))

It's been a long day so that'll need checking in the morning:-)

Thanks for the ideas,

--Beman